The human brain has been the object of many methodologies and hypotheses. The burgeoning technologies now available have each opened particular facets of the study, MRI being the most versatile as capable of imaging hemodynamic and metabolic signals in a unique fashion. PET has similar possibilities of large chemical specificity governed by the combination of lifetimes and radiation from radioactive isotopes. Other methods give highly specialized signals, for example, MEG and EEG that have respectively high and low resolution for neurophysiological signals. Optical tomography is somewhat more quantitative with respect to hemodynamic changes and has latent possibilities for measuring neuronal signals. The propagation of near infrared light through biological tissue such as the brain and breast has been experimentally studied and theoretically modeled. Accurate theoretical models are based on Monte Carlo representations of the diffusion equation and on analytic expressions that show propagation into the gray matter of the brain in adults and especially in neonates. This propagation of light into cranial tissue has been verified by clinical measurement of the presence of X-ray CT identified cranial hematomas at depths 3-4 cm. Detection of the oxygenation state and amount of hemoglobin has been the goal of tissue oximetry and quantitative results are obtained by time and frequency domain devices. These devices measure the optical pathlength over which photons diffuse from source to detector. However, single volume determination of optical parameters of a highly heterogeneous system such as the human brain may give only a fraction of the signal of a localized focal activation already shown to be highly localized by ƒMRI (functional Magnetic Resonance Imaging).
The optical systems are relatively simple, safe, portable and affordable as required by today's health care industry. There are several optical examination and imaging devices that have been used for imaging functional activity of adult, full-term and pre-term neonate brain. These optical examination and imaging systems are described in U.S. Pat. Nos. 5,353,799; 5,853,370; 5,807,263, 5,820,558. The described optical systems do not require subject immobilization (as do MRI and PET). The images are acquired in less than half a minute and show good two-dimensional resolution of blood changes.
The amplitude and phase cancellation devices have been used to study the prefrontal region of adults and the parietal region of pre-and full-term infants and have been used for the detection of breast tumors. In addition to the use of phase and amplitude cancellation, there are two general principles for object detection in the biological tissue. In brain examination, an optical system can use the difference between rest and functional activation since the transition time between the two states of the brain function is several seconds. The second principle is used, where symmetry exists such as in the hemispheres of the brain or in the human breasts. The symmetric “lateralization” signal has been used to detect brain bleeds and breast tumors since it enables cancellation unwanted background signals.
Phase cancellation systems use preferably an arrangement of detectors and equidistant sources from which amplitude and phase cancellation may be obtained. If equal light amplitudes at 0 and 180° phases are used for the light sources, an appropriate positioning of the detector can lead to a “null” in the amplitude signal and a crossover between 0 and 180° phase shift, i.e., 90°. In homogeneous tissue, this null is symmetrically in the middle. In heterogeneous tissue, this null may is displaced from the geometric midline. Thus, the null establishes an extremely sensitive measure to perturbation by an absorber or scatterer. The detection sensitivity can be further increased by using a NIR absorbing dye such as indocyanine green (ICG) administered or injected in the tissue. Advantageously, the null location is relatively insensitive to amplitude fluctuations common to both light sources, and is insensitive to inhomogeneities that affect a large tissue volume common to the two optical patterns. Sensitivity to scattering is high particularly provided that the scattering contrast is the same as the absorbing contrast. The crossover signal is preferably used for imaging. The amplitude signal is somewhat less useful for imaging since the position indication is ambiguous, i.e., an increase of signal is observed regardless of the displacement of the absorbing object away from to the null plane. However, this can be accounted for by software corrections.
The optical phase system generates a photon diffusion wave of a long wavelength (˜10 cm) as determined by the particular scattering (μs′=10 cm−1) and absorbing (μa=0.04 cm−1) properties of the medium and the radio-frequency (˜100 MHz). Optical wavelengths in the visible to infrared range, preferably between 700-850 nm are determined by the availability of laser diodes. Thus the photon diffusion wavelength of ˜10 cm provides imaging in the “near field”, and diffraction effects are absent. The phase signal at zero crossing detection is essentially a square wave “overloaded” signal. It is moderately insensitive to the changes of signal amplitude that may occur in imaging from proximal to distal source-detector pairs and is also little insensitive to ambient light.
There is still a need for optical examination and imaging systems for examining various brain cognitive functions and processes.